C4 is cleaved by activated protease C1s in the classical pathway or MASPs in the lectin-mediated pathway producing two fragments C4a and C4b. The larger fragment C4b binds covalently to the target surface due to a highly reactive thioester bond which becomes exposed upon cleavage. Chicken C4 shares 37% amino acid sequence identity with its human counterpart. Correlation between chicken MBL concentration and C4 deposition was observed in chicken sera [Norup LR and Juul-Madsen HR 2007].
The exact cleavage site in chicken C4 is unknown.